(a) Field of the Invention
This invention generally relates to a long stroke pumping apparatus for oil wells and the like and more particularly to a multi-cable universal long stroke pumping apparatus.
(b) Description of the Prior Art
A beam-type pumper is generally limited to a stroke length of about 16 to 18 feet. The string of sucker rods accelerate from the start of each stroke to the middle and decelerate from there to the end of the stroke. It was long recognized that a longer stroke and a slower but constant speed for the sucker rods would be an improvement. Efforts have therefore been made to build long-stroke pumpers. For example, U.S. Pat. Nos. 3,285,081 and 3,528,305 describe one such pumper.
Another long-stroke pumper is described in the November, 1976 issue of World Oil, Pages 64-68. It is cable-operated by an electronically-controlled electric motor. It utilizes a drum on which a first cable is wound in one direction to raise and lower the rod string. A second cable also connected to the drum is pulled in the opposite direction by a counterweight which partially balances the torque load required to lift the rod string and the production fluid from the well. The steel wire cables wound on the drum have a large diameter and require large diameter sheaves. The drum itself is rotated by a high-torque gear box driven by the electric motor which is mechanically switched to operate in forward and reverse for up and down strokes. The drum's surface is provided with spiraling grooves, or cams, which cause the cables to follow paths that change the radius of the drum near the end of each stroke, thereby altering respective torque loads on the well-side and on the counterweight-side cables. As a result, the counterweight-torque is decreased by a smaller radius and the upward motion will stop and reverse. Similarly, at the bottom of the stroke, the well-side torque is decreased by a smaller radius and the downward motion will stop and reverse. During a portion of this pendulum type motion, the electric motor is switched off, otherwise it is required to supply power over most of the length of each stroke. The direction of rotation of the motor is controlled by conventional mechanical reversing contactors that are actuated by an electronic system. At the proper time during each stroke, the motor is switched off or on as required.
The great number of such required on-and-off switching operations produce wear and tear. The stroke length and the number of strokes can be adjusted but over a relatively limited range of about .+-.10%.
While the prior art cable-operated long stroke pumpers constitute an improvement over the conventional beam-type pumpers, nevertheless they are still characterized by drawbacks chief among which are: they require large diameter drums, sheaves and cables; their cables become subjected to excessive wear; their electric prime movers require high torque gear boxes to rotate their drums and no other type prime movers can be employed; their physical dimensions and weights, while reduced as compared to the beam type pumpers, are still excessively large to make full enclosures practical, hence excessive wear on the pumper units; they are inherently restricted to the limited operational range they are designed for; their velocity and acceleration are not easily programmable; they are insensitive to downhole problems and will continue to work until a sensed overload will set an electronic brake, or until a catastrophic failure occurs; and should their single cable break on the wellside, the rods which will accelerate downwardly will buckle, the downhole pump will most likely become destroyed, and the production tubing damaged.
It is therefore a general object of the present invention to provide a universal long-stroke pumper (ULSP) which retains the advantages of prior art long-stroke pumpers while eliminating their above described and other known disadvantages.